Why do rulers play by the rules? We show that the legality requirement under the rule of law implements an endogenous enforcement mechanism supporting constitutionality. Agents which do not obey unconstitutional legal norms are not sanctioned under constitutional rule. A principal who defects from the constitution but cannot commit himself to never reinstall the constitution finds law enforcement more difficult. As more agents disobey, enforcement becomes less effective. The expectation of an eventual return to constitutionality becomes self-fulfilling. We show this mechanism to be effective in deterring a government from violating constitutional norms.